In hGH radioreceptor assay unlabeled hGH competes with 125I-hGH for binding a fixed number of receptor sites on cultured human lymphocytes (IM-9). The ability of hGH to regulate its receptor concentration has been exploited to increase the sensitivity of the hGH in radioreceptor assay. In general, radioimmunoassays are more sensitive than receptor assay than in vivo bioassays. The specificities of the various assays are different and these studies focus on the binding of hGH to its receptor.